This invention relates generally to aerodynamic decelerators and more particularly to the aerodynamic stabilization of parachutes having square canopies.
When dropping a store such as a sonobuoy into the ocean from a high altitude it is necessary to assure proper orientation upon impact. This requires the use of a parachute which has high drag to slow the store's descent and stability to minimize its oscillations. Parachute instability, caused by destabilizing aerodynamic side forces on the canopy, is translated to the store through the parachute's suspension lines, causing the store to oscillate, adversely affecting impact orientation. An air-deployed payload which has a video camera mounted therein for transmission during air descent also requires a parachute with high drag efficiency and extreme stability to prevent oscillation of the payload during descent. Since a parachute's stability and drag efficiency are inversely related, parachutes with sufficient drag efficiency for these applications often lack sufficient stability, and vice versa.
Additionally, many of the currently used parachutes which provide either high drag efficiency or good stability have a complex design and are therefore difficult and expensive to manufacture. For example, the guide surface parachute, shown in U.S. Pat. No. 2,683,575 to Heinrich, is the most stable parachute available today. Although very stable, its drag efficiency is poor and it has a complex geometry. Other parachutes, such as the cross-type, require from 8 to 12 suspension lines to provide sufficient drag for the applications mentioned above, adding not only to cost and packing complexity, but also to the potential for entanglement problems.
Attempts to simplify parachute design and yet maintain sufficient drag efficiency have led to the use of parachutes with square canopies. Such canopies, although simple, are inherently unstable. Stability has been improved by reducing the ratio of suspension line length to nominal canopy diameter, and by using different suspension line arrangements, such as suspension line shapers. Further enhancements have been achieved by providing carefully located slots in the square canopy. Although these modifications are effective in improving the stability of the square parachute, a parachute with a greater stabilizing moment than the guide-surface parachute is desired, without a significant degradation in drag or design simplicity.